It’s paradise with a twist. As the sun sets over the beautiful Palmyra atoll, south of Hawaii, Earth’s largest land arthropod emerges from its lair.

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Coconut crabs can reach a length of 40 centimetres, with a leg span of 90 centimetres, and weigh 4 kilograms. Most such monster arthropods – the group that includes insects, spiders and crustaceans – live in the sea, where the water helps support their heavy bodies. To survive on the land, coconut crabs have had to evolve a suite of strange adaptations. But their exceptional lifestyle has also put them at great risk, and conservationists are only now working out how to protect them.

They begin their lives in the sea. Female crabs carry their fertilised eggs around on their bodies, and release them into the sea when they hatch – generally when the moon is new. The juveniles develop in the sea for about a month, after which they head out onto land.

There they face a problem&colon; how to breathe. Aquatic crabs have gills, but these are inefficient out of the water. To get round this, coconut crabs have developed organs called branchiostegal lungs, which are essentially sets of gills turned inside out. They have only been able to develop these strange organs because, unlike other hermit crabs, they perform the slightly obscene-sounding trick of exposing their abdomens to the surrounding air.

Nuts for coconuts

As their name suggests they eat coconuts and nuts, using their strong claws to tear open the tough outer shells. In some cases the crabs may carry coconuts into trees and then drop them to break them open (PDF).

Rather than mouths, crabs have a tube called the gastric mill for “chewing” food. In coconut crabs, this has a tough flattened surface against which hard food like nuts can be crushed.

Staying alive

Coconut crabs have been enthusiastically hunted – not least because people tend to eat them – and in some cases wiped out on many islands. Although the International Union for Conservation of Nature’s Red List of endangered species classes them as “data deficient“, because the populations have not been studied recently, it is clear that the population has been slashed.

A captive breeding programme to restock the islands is under way, with some early success&colon; last year it was reported that juvenile crabs have been successfully taken through all their stages of development in the lab. But this is only half the solution.

Because coconut crabs live both in the sea and on the land, any attempt to protect them has to cover both habitats, and ensure that they can move easily between them. Most conservation programmes do not allow for this sort of habitat switching, focusing instead on protecting one particular habitat such as a coral reef.

However, Maria Beger of the University of Queensland in Brisbane, Australia, and colleagues have proposed new systems for protecting animals that need to move between habitats. Their ideas have not yet been tried out in the wild, but if implemented they could give animals like the coconut crab a much better chance.